Output circuits with class D amplifier

ABSTRACT

Output circuits using pulse width modulation (PWM) and/or pulse density modulation (PDM) are described. In one aspect, a PWM output circuit includes a PWM modulator that operates based on a square wave signal instead of a sawtooth or triangular wave signal. In another aspect, a PDM output circuit includes a PDM modulator that uses variable reference voltages to reduce variations in switching frequency. In yet another aspect, a dual-mode output circuit supports both PWM and PDM and includes a pulse modulator and a class D amplifier. The pulse modulator performs PWM on an input signal if a PWM mode is selected and performs PDM on the input signal if a PDM mode is selected. The class D amplifier receives a driver signal from the pulse modulator and generates an output signal.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a divisional of patent applicationSer. No. 11/748,828 entitled “OUTPUT CIRCUITS WITH CLASS D AMPLIFIER”filed May 15, 2007, allowed, and assigned to the assignee hereof andhereby expressly incorporated by reference herein.

BACKGROUND

I. Field

The present disclosure relates generally to electronics circuits, andmore specifically to output circuits for electronics devices.

II. Background

Output amplifiers are commonly used in various applications such asaudio power amplifiers, telephone line drivers, etc. Output amplifiersmay be categorized into different classes such as class A, class B,class AB and class D. Class A, B and AB amplifiers are linear amplifiersthat operate in linear region. Class D amplifiers are switch modeamplifiers that operate in triode and cut-off regions. Consequently,class D amplifiers can typically achieve much higher power efficiencythan linear amplifiers.

A class D amplifier is commonly used with a modulation technique such aspulse width modulation (PWM) or pulse density modulation (PDM). A PWMmodulator may receive an analog input signal and a carrier signal andgenerate a PWM signal that controls the switching of the class Damplifier. The PWM signal has a variable duty cycle determined by theamplitude of the analog input signal and a fixed switching frequencydetermined by the frequency of the carrier signal. The carrier signalmay have a sawtooth waveform or a triangular waveform. Any deviationfrom an ideal sawtooth or triangular waveform may cause distortion in anoutput signal from the class D amplifier.

A PDM modulator may receive an analog input signal and generate a PDMsignal that controls the switching of the class D amplifier. The PDMsignal has a duty cycle determined by the analog input signal and mayhave a variable switching frequency determined by circuit componentswithin the PDM modulator. The values of the circuit components may varywidely due to integrated circuit (IC) process variations and otherfactors. Hence, the switching frequency of the PDM signal may also varywidely.

SUMMARY

Output circuits using pulse width modulation (PWM) and/or pulse densitymodulation (PDM) and having improved performance/characteristics aredescribed herein. In an aspect, a PWM output circuit includes a PWMmodulator that operates based on a square wave signal instead of asawtooth or triangular wave signal. In one design, the PWM outputcircuit includes an integrator and a comparator that collectivelyperform PWM on an input signal and a class D amplifier that generates anoutput signal. The integrator sums and integrates the input signal, thesquare wave signal, and the output signal and provides an integratedsignal. The comparator compares the integrated signal against a fixedreference voltage and generates a driver signal. The class D amplifierreceives the driver signal and generates the output signal. A clockdivider may receive and divide a clock signal in frequency to generatethe square wave signal.

In another aspect, a PDM output circuit includes a PDM modulator thatuses variable reference voltages to reduce variations in switchingfrequency. In one design, the PDM output circuit includes an integratorand a comparator that collectively perform PDM on an input signal, aclass D amplifier that generates an output signal, and a referencegenerator. The integrator sums and integrates the input and outputsignals and provides an integrated signal. The comparator compares theintegrated signal against adjustable reference voltages and generates adriver signal. The class D amplifier receives the driver signal andgenerates the output signal. The reference generator generates theadjustable reference voltages such that variations in the switchingfrequency of the driver signal is reduced. For example, the referencegenerator may generate first and second reference voltages having adifference that is inversely related to a feedback resistor in theintegrator, related to a power supply voltage, etc.

In yet another aspect, a dual-mode output circuit supports both PWM andPDM. In one design, the dual-mode output circuit includes a pulsemodulator and a class D amplifier. The pulse modulator performs PWM onan input signal if a PWM mode is selected, performs PDM on the inputsignal if a PDM mode is selected, and provides a driver signal. Thepulse modulator may include an integrator and a comparator. Theintegrator sums and integrates the input signal and an output signal(and also a square wave signal in the PWM mode) and provides anintegrated signal. The comparator compares the integrated signal againsta fixed reference voltage in the PWM mode and against first and secondreference voltages in the PDM mode. The class D amplifier receives thedriver signal from the comparator and generates the output signal.

Various aspects and features of the disclosure are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an open-loop PWM output circuit.

FIG. 2 shows a closed-loop PWM output circuit.

FIG. 3 shows another closed-loop PWM output circuit.

FIG. 4 shows a closed-loop PWM output circuit using a square wavesignal.

FIG. 5 shows a closed-loop PDM output circuit.

FIG. 6 shows a closed-loop PDM output circuit with adjustable referencevoltages.

FIG. 7 shows a closed-loop dual-mode output circuit.

FIG. 8 shows a differential closed-loop dual-mode output circuit.

FIG. 9 shows a reference generator.

FIG. 10 shows a process for operating an output circuit.

FIG. 11 shows a wireless communication device with at least one outputcircuit.

DETAILED DESCRIPTION

FIG. 1 shows a schematic diagram of an open-loop PWM output circuit 100driving an output load 160. PWM output circuit 100 includes a PWMmodulator 110 and a class D amplifier 150.

Within PWM modulator 110, a comparator (Comp) 132 receives an analoginput signal V_(in) at a non-inverting input and a carrier signalV_(car) at an inverting input, compares the two signals, and provides aPWM driver signal. A carrier generator 134 generates the carrier signalat a fixed frequency. The carrier signal may have a sawtooth waveform, atriangular waveform, etc. The PWM driver signal is digital in nature andtoggles between logic high (e.g., a power supply voltage V_(dd)) andlogic low (e.g., circuit ground). The PWM driver signal has a variableduty cycle determined by the amplitude of the analog input signal and afixed switching frequency determined by the frequency of the carriersignal.

Within class D amplifier 150, a driver 152 receives the PWM driversignal and generates a first control signal C1 for a P-channel fieldeffect transistor (P-FET) 154 and a second control signal C2 for anN-channel FET (N-FET) 156. Driver 152 generates the first and secondcontrol signals such that only P-FET 154 or N-FET 156 is turned on atany given moment and the P-FET and N-FET are turned on and off atappropriate times. P-FET 154 has its source coupled to the power supplyV_(dd), its gate receiving the first control signal, and its draincoupled to the drain of N-FET 156. N-FET 156 has its gate receiving thesecond control signal and its source coupled to circuit ground. Thedrains of FETs 154 and 156 provide an output signal V_(out) for class Damplifier 150.

Within output load 160, a filter 162 receives and filters the outputsignal from class D amplifier 150 and provides a filtered signal to aload 164. Filter 162 may be a lowpass filter, a bandpass filter, etc.Filter 162 may also perform DC blocking Load 164 may be a speaker (asshown in FIG. 1) or some other type of load.

PWM output circuit 100 operates as follows. Comparator 132 generates apulse in the PWM driver signal for each period or cycle of the carriersignal. The width of the pulse is determined by the amplitude of theanalog input signal, which may be considered as providing a variablereference against which the carrier signal is compared. The PWM driversignal contains a stream of pulses. Driver 152 generates the first andsecond control signals based on the PWM driver signal. Ideally, when thePWM driver signal is at logic high, P-FET 154 is turned off, N-FET 156is turned on, and the output signal is pulled low. Conversely, when thePWM driver signal is at logic low, P-FET 154 is turned on, N-FET 156 isturned off, and the output signal is pulled high. The second controlsignal may resemble the PWM driver signal, and the first control signalmay resemble an inverted version of the PWM driver signal. The first andsecond control signals may also be generated to achieve a desired amountof non-overlap between the two control signals to obtain the desiredoutput signal characteristics.

The output signal is digital in nature and includes harmonics of thecarrier/switching frequency. Filter 162 filters the output signal topass the desired signal component and remove extraneous high frequencycomponents. Since the switching frequency is typically much higher thanthe bandwidth of the desired signal, filter 162 may be implemented witha simple low-order filter, e.g., an LC filter composed of an inductor(L) and a capacitor (C).

PWM output circuit 100 has several drawbacks. First, the total harmonicdistortion (THD) of the output signal is dependent on the fidelity ofthe carrier signal from generator 134. Any deviation from an idealsawtooth or triangular waveform may result in the PWM driver signalcontaining pulses with inaccurate duty cycles, which may then causedistortion in the output signal. Second, noise on the power supply iscoupled to the output signal. The power supply rejection ratio (PSRR) isapproximately 6 decibels (dBs), which means that the noise in the powersupply is attenuated by only one half in the output signal.

FIG. 2 shows a schematic diagram of a closed-loop PWM output circuit102, which includes a PWM modulator 112 and class D amplifier 150.Within PWM modulator 112, an amplifier (Amp) 122 receives an analoginput signal V_(in) at a non-inverting input and a feedback signal at aninverting input and provides an amplified signal. The feedback signal isequal to an output signal V_(out) from class D amplifier 150. A filter118 filters the amplified signal with a filter response and provides afiltered signal to the inverting input of a comparator 132. Comparator132 also receives a carrier signal V_(car) at its non-inverting inputand provides a PWM driver signal to driver 152 within class D amplifier150.

FIG. 3 shows a schematic diagram of a closed-loop PWM output circuit104, which includes a PWM modulator 114 and class D amplifier 150. PWMmodulator 114 includes a summer integrator 120 followed by a comparatorsection 130. Summer integrator 120 performs summing function for theclosed-loop feedback and also implements filter 118 in FIG. 2.

Integrator 120 includes amplifier 122, resistors 124 and 128, and acapacitor 126. Resistor 124 has one end coupled to an inverting input ofamplifier 122 and the other receiving an analog input signal V_(in).Capacitor 126 has one end coupled to the inverting input of amplifier122 and the other end coupled to an output of the amplifier. Resistor128 has one end coupled to the inverting input of amplifier 122 and theother end coupled to the output of class D amplifier 150. Amplifier 122has its non-inverting input coupled to a reference voltage V_(ref) andits output coupled to the non-inverting input of comparator 132.Comparator 132 receives a carrier signal V_(car) at its inverting inputand provides a PWM driver signal to driver 152 within class D amplifier150.

Resistor 128 provides feedback from the output of class D amplifier 150to PWM modulator 114. Integrator 120 sums the analog input signal andthe feedback signal and further integrates the summed signal to generatean integrated signal V_(int). The integrated signal includes anintegrated analog input signal and an integrated feedback signal. Theintegrated signal is provided to and operated on by comparator 132 andclass D amplifier 150 in the same manner as comparator 132 and class Damplifier 150 in FIG. 1.

The use of feedback in PWM output circuit 104 improves both THD andPSRR. The amount of improvement in THD and PSRR is dependent on (a) aforward gain determined by components of integrator 120 and (b) afeedback gain determined by resistors 124 and 128. However, a sawtoothor triangular carrier signal is still used for comparator 132.

In an aspect, a PWM output circuit includes a PWM modulator thatoperates based on a square wave signal instead of a sawtooth ortriangular wave signal. The square wave signal may be much easier togenerate than the sawtooth or triangular wave signal.

FIG. 4 shows a schematic diagram of a closed-loop PWM output circuit 106that uses a square wave signal. PWM output circuit 106 includes a PWMmodulator 116 and class D amplifier 150. PWM modulator 116 includescircuit elements/components 122 through 132 that are coupled asdescribed above for PWM modulator 114 in FIG. 3. However, the invertinginput of comparator 132 is coupled to a reference voltage V_(ref)instead of carrier generator 134. PWM modulator 116 further includes aclock divider 140 and a resistor 142. Clock divider 140 receives a clocksignal, divides the clock signal in frequency, and provides a squarewave signal V_(sq) with 50% duty cycle. Resistor 142 has one end coupledto clock divider 140 and the other end coupled to the inverting input ofamplifier 122.

Integrator 120 sums the analog input signal, the feedback signal, andthe square wave signal and further integrates the summed signal togenerate an integrated signal V_(int). The integrated signal includesthe integrated analog input signal, the integrated feedback signal, andthe integrated square wave signal. Comparator 132 compares theintegrated signal from integrator 120 against the reference voltage andgenerates a PWM driver signal.

PWM modulator 116 uses a square wave injected at the input of integrator120 instead of a sawtooth or triangular wave at the input of comparator132. Integrator 120 converts the square wave into a triangular wave andadds it to the integrated signal. Resistor 128 in the feedback path maybe implemented with the same type of resistor (e.g., fabricated with thesame IC processing steps) as resistor 124 in the forward path for theanalog input signal. In this case, the feedback and forward pathsautomatically track one another over IC process, temperature, and powersupply (PVT) variations.

In general, any suitable values may be selected for the components ofPWM modulator 116. In one design, resistor 142 has a value that is onehalf of the value of feedback resistor 128. This choice of resistorvalues provides a 2 to 1 ratio in the slopes of the integrated squarewave signal to the integrated feedback signal. Other values may also beselected for resistors 124, 128 and 142 and capacitor 126.

The use of a square wave signal may simplify the design of PWM outputcircuit 106. A square wave with 50% duty cycle and toggling betweenlogic high and logic low may be easily generated based on any clocksignal commonly available in an electronics device. It is generallyeasier to obtain 50% duty cycle square wave than good fidelity forsawtooth or triangular wave. Furthermore, clock divider 140 is typicallymuch easier to implement than a sawtooth or triangular wave generator.

FIG. 5 shows a schematic diagram of a closed-loop PDM output circuit 500driving an output load 560. PDM output circuit 500 includes a PDMmodulator 510 and a class D amplifier 550.

Within PDM modulator 510, an amplifier 522 receives an analog inputsignal V_(in) at a non-inverting input and a feedback signal at aninverting input and provides an amplified signal. The feedback signal isequal to an output signal V_(out) from class D amplifier 550. A filter530 filters the amplified signal with a filter response and provides afiltered signal. A comparator 532 receives the filtered signal at aninverting input and a reference signal at a non-inverting input,compares the two signals, and provides a PDM driver signal. A referencegenerator 534 generates the reference signal as described below. The PDMdriver signal has a density determined by the analog input signal and aswitching frequency determined by various components such as filter 530.

Class D amplifier 550 includes a driver 552, a P-FET 554, and an N-FET556 that are coupled in the same manner as driver 152, P-FET 154, andN-FET 156, respectively, in FIGS. 1 through 4. Output load 560 includesa filter 562 and a load 564 that are coupled in the same manner asfilter 162 and load 164, respectively, in FIGS. 1 through 4.

Generator 534 generates reference voltages V_(L) and V_(H) for thereference signal provided to comparator 532. A multiplexer (Mux) 536receives the two reference voltages from generator 534, provides the lowreference voltage V_(L) to comparator 532 when the PDM driver signal isat logic high, and provides the high reference voltage V_(H) when thePDM driver signal is at logic low. The use of two reference voltagesprovides hysteresis for comparator 532.

A loop is formed by amplifier 522, filter 530, comparator 532 and classD amplifier 550. The loop is designed to be unstable when no analoginput signal is applied and to oscillate at a particular switchingfrequency. This switching frequency is determined by the response offilter 530, the high and low reference voltages provided to comparator532, and the propagation delay through comparator 532 and class Damplifier 550.

PDM output circuit 500 operates as follows. When no analog input signalis applied, self-oscillation of the loop results in the PDM driversignal from comparator 532 containing square waves at the switchingfrequency. This self-oscillation occurs as follows. If the output ofcomparator 532 is at logic low, then V_(out) is low, and V_(H) isprovided to the inverting input of comparator 532. If V_(in) is atmidscale, then the output of amplifier 522 goes high, and the output offilter 530 transitions high at a rate determined by the response of thefilter. After the filtered signal exceeds V_(H), the output ofcomparator 532 transitions to logic high, V_(out) goes high, and V_(L)is provided to the inverting input of comparator 532. The output ofamplifier 522 then goes low, and the output of filter 530 transitionslow at a rate determined by the response of the filter. After thefiltered signal drops below V_(L), the output of comparator 532transitions to logic low, V_(out) goes low, and V_(H) is provided to theinverting input of comparator 532. The output of amplifier 522 then goeshigh, and a new switching cycle starts. One period of the switchingfrequency is thus dependent on (a) the response of filter 530, whichdetermines how fast the filtered signal transitions high and low, (b)the reference voltages V_(H) and V_(L), which determine how much thefiltered signal needs to transition to cause comparator 532 to switch,and (c) other components that determine the delay through the loop.

When the analog input signal is applied, the PDM driver signal fromcomparator 532 contains pulses with variable widths determined by theanalog input signal. The square wave from the loop self-oscillation actsas dither. The PDM driver signal contains the analog input signal andthe switching frequency.

FIG. 6 shows a schematic diagram of a closed-loop PDM output circuit502, which includes a PDM modulator 512 and class D amplifier 550. PDMmodulator 512 implements filter 530 in FIG. 5 with a summer integrator520. Integrator 520 includes amplifier 522, resistors 524 and 528, andcapacitor 526 that are coupled in the same manner as amplifier 122,resistors 124 and 128, and a capacitor 126, respectively, in FIG. 3.

For PDM output circuit 502, one period of the switching frequency may beexpressed as:

$\begin{matrix}{{T_{sw} = {\frac{4}{1 - {4 \cdot \left( {V_{i\; n}/V_{dd}} \right)^{2}}} \cdot \left( {{\Delta\; t} + \frac{{R_{fb} \cdot C \cdot \Delta}\; V}{V_{dd}}} \right)}},} & {{Eq}\mspace{14mu}(1)}\end{matrix}$where T_(sw) is one period of the switching frequency,

C is the value of integrator capacitor 526,

R_(fb) is the value of feedback resistor 528,

V_(mid) is a midscale reference voltage, which isV_(mid)≈(V_(H)+V_(L))/2,

ΔV is a comparator window size, which is ΔV=V_(H)−V_(L), and

Δt is the propagation delay from comparator 532 to the output signal.

As shown in equation (1), the switching period is dependent on variousparameters. The values of some of these parameters may vary widely. Forexample, the feedback resistor R_(fb) may vary by more than 50% acrossIC process, voltage, and temperature (PVT). The integrator capacitor Cmay also vary by a large amount across PVT. The power supply voltageV_(dd) may range, e.g., from 3.3 to 4.4 Volts. The switching frequencymay thus vary widely due to large variations in one or more of theparameters in equation (1).

In another aspect, variable reference voltages are used to account orcompensate for variations in one or more parameters affecting theswitching frequency. The second term R_(fb)·C·ΔV/V_(dd) in the secondpart of equation (1) may be much larger than the first term Δt. Hence,the reference voltages may be generated such that the termR_(fb)·C·ΔV/V_(dd) has small variation. This may be achieved bygenerating the comparator window size ΔV, as follow:

$\begin{matrix}{{{\Delta\; V} = \frac{K \cdot V_{dd}}{R_{fb} \cdot C}},} & {{Eq}\mspace{14mu}(2)}\end{matrix}$where K is a constant.

As shown in equation (2), to obtain small variation inR_(fb)·C·ΔV/V_(dd), the comparator window size ΔV may be generated suchthat it is proportional to the power supply voltage V_(dd) and inverselyproportional to the feedback resistance R_(fb) and the integratorcapacitance C. A translinear circuit may be used to generate thereference voltages V_(L) and V_(H) to adaptively track variations inV_(dd), R_(fb) and C, or any combination of these parameters over PVT.Small variation in R_(fb)·C·ΔV/V_(dd) due to use of an adaptive ΔV mayresult in small variation in the switching frequency, which isdesirable.

The reference voltages may also be generated to account for variationsin the analog input signal and/or other parameters. This may be achievedby generating the comparator window size ΔV, as follow:

$\begin{matrix}{{\Delta\; V} = {{\frac{K \cdot V_{dd}}{R_{fb} \cdot C} \cdot \left\lbrack {1 - \left( {V_{i\; n}/V_{mid}} \right)^{2}} \right\rbrack} - {\frac{\Delta\;{t \cdot V_{dd}}}{R_{fb} \cdot C}.}}} & {{Eq}\mspace{14mu}(3)}\end{matrix}$The term to the right of the minus sign in equation (3) may be ignoredif Δt is small.

As shown in equation (3), the comparator window size ΔV may be generatedsuch that it is proportional to [1−(V_(in)/V_(mid))²] and follows theinput signal in real time.

The use of feedback in PDM output circuit 502 improves both THD andPSRR. The amount of improvement in THD and PSRR is dependent on theforward and feedback gains. The use of adaptive reference voltagesreduces variations in the switching frequency of PDM modulator 512. Allof these features may improve the performance of PDM output circuit 502.

In yet another aspect, a dual-mode output circuit includes a pulsemodulator that supports both PWM and PDM. As shown in FIGS. 4 and 6, PWMoutput circuit 106 in FIG. 4 closely resembles PDM output circuit 502 inFIG. 6. This is achieved by (a) using a summing integrator for thefeedback signal and (b) injecting a square wave signal into theintegrator instead of a sawtooth or triangular wave signal into thecomparator. The dual-mode output circuit may be efficiently implementedwith mostly circuits that are common to both PWM and PDM and someadditional circuits that are specific to PWM or PDM.

FIG. 7 shows a schematic diagram of a closed-loop dual-mode outputcircuit 700 driving an output load 760. Dual-mode output circuit 700includes a pulse modulator 710 and a class D amplifier 750. Pulsemodulator 710 can operate in either a PWM mode or a PDM mode, which maybe selected via a Mode control signal.

Within pulse modulator 710, an amplifier 722, resistors 724 and 728, anda capacitor 726 of an integrator 720, a comparator 732, a referencegenerator 734, and a multiplexer 736 are coupled as described above forPDM output circuit 502 in FIG. 6. Generator 734 generates threereference voltages V_(ref), V_(L), and V_(H). In the PWM mode,multiplexer 736 provides reference voltage V_(ref) of comparator 732. Inthe PDM mode, multiplexer 736 provides either the low reference voltageV_(L) or the high reference voltage V_(H) to comparator 732 at any givenmoment depending on the output of the comparator. Pulse modulator 710further includes a clock divider 740, a resistor 742, and a switch 744.Clock divider 740 receives a clock signal and generates a square wavesignal in the PWM mode. Resistor 742 has one end coupled to the outputof clock divider 740 and the other end coupled to one end of switch 744.The other end of switch 744 is coupled to the inverting input ofamplifier 722. Switch 744 is closed in the PWM mode and opened in thePDM mode. Table 1 summarizes the operation of switch 744 and referencegenerator 734 for the PWM and PDM modes.

TABLE 1 Mode Switch 744 Multiplexer 736 PWM closed V_(ref) PDM openV_(L) and V_(H)

FIGS. 1 through 7 show singled-ended PWM, PDM and dual-mode outputcircuits. Differential PWM, PDM and dual-mode output circuits may alsobe designed and used to achieve certain benefits such as improvedlinearity, dynamic range, PSRR, and rejection of common mode signals,greater output power since the voltage applied to the load may bedoubled, etc.

FIG. 8 shows a schematic diagram of a differential closed-loop dual-modeoutput circuit 800 driving an output load 860. Differential dual-modeoutput circuit 800 includes a pulse modulator 810 and a class Damplifier 850. Pulse modulator 810 can operate in either a PWM mode or aPDM mode.

Pulse modulator 810 includes a non-inverting path composed of anamplifier 822 a, resistors 824 a, 828 a and 842 a, a capacitor 826 a, aswitch 844 a, and a comparator 832 a that are coupled as described abovefor amplifier 722, resistors 724, 728 and 742, capacitor 726, switch 744and comparator 732, respectively, in FIG. 7. Pulse modulator 810 furtherincludes an inverting path composed of an amplifier 822 b, resistors 824b, 828 b and 842 b, a capacitor 826 b, a switch 844 b, and a comparator832 b that are coupled in the same manner as amplifier 822 a, resistors824 a, 828 a and 842 a, capacitor 826 a, switch 844 a, and comparator832 a, respectively. Resistor 824 a receives a non-inverting inputsignal V_(in+), and resistor 824 b receives an inverting input signalV_(in−). Resistors 844 a and 874 b both couple to a clock divider 840that provides a square wave signal. The non-inverting inputs ofcomparators 832 a and 832 b couple to the outputs of multiplexers 836 aand 836 b, respectively. A reference generator 834 generates threereference voltages V_(ref), V_(L), and V_(H). In the PWM mode,multiplexers 836 a and 836 b provide the reference voltage V_(ref) tocomparators 832 a and 832 b, respectively. In the PDM mode, multiplexer836 a provides the low reference voltage V_(L) to comparator 832 a whenthe comparator output is at logic high and provides the high referencevoltage V_(H) to comparator 832 a when the comparator output is at logiclow. Similarly, multiplexer 836 b provides either the low or highreference voltage to comparator 832 b at any given moment. In the PDMmode, the low reference voltage V_(L) is provided to one comparator andthe high reference voltage V_(H) is provided to the other comparator atany given moment.

FIG. 9 shows a schematic diagram of a design of a reference generator900, which may be used for reference generators 534, 634, 734 and 834 inFIGS. 5, 6, 7 and 8, respectively. Within reference generator 900,resistors 912, 914 and 916 are coupled in series and between the powersupply voltage V_(dd) and circuit ground. Resistors 912, 914 and 916 aredesigned such that the ratio of the value of resistor 914 to the totalvalue of all three resistors is α. The voltage across resistor 914 isthus a α·V_(dd). A gm cell (transconductance amplifier) 920 has itsdifferential input coupled across resistor 914, its bias input coupledto a current source 922, and its differential output coupled to thedifferential input of a translinear gain cell 930. Gain cell 930 has itsbias input coupled to a current source 932 and its differential outputcoupled to resistors 940 and 942 and providing the reference voltagesV_(H) and V_(L), respectively. Resistors 940 and 942 are further coupledto the power supply voltage V_(dd). Current source 922 provides a biascurrent of 2I₁ for gm cell 920, and current source 932 provides a biascurrent of 2I₂ for gain cell 930. gm cell 920 has a gain of1/(β·R_(fb)).

The currents at the differential output of gm cell 920 may be expressedas:

$\begin{matrix}{I_{a} = {{I_{1} - {\frac{\alpha \cdot V_{dd}}{\beta \cdot R_{fb}}\mspace{14mu}{and}\mspace{14mu} I_{b}}} = {I_{1} + {\frac{\alpha \cdot V_{dd}}{\beta \cdot R_{fb}}.}}}} & {{Eq}\mspace{14mu}(4)}\end{matrix}$

The currents at the differential output of gain cell 930 may beexpressed as:

$\begin{matrix}{I_{c} = {{I_{2} + {\frac{I_{2} \cdot \alpha \cdot V_{dd}}{I_{1} \cdot \beta \cdot R_{fb}}\mspace{14mu}{and}\mspace{14mu} I_{d}}} = {I_{2} - {\frac{I_{2} \cdot \alpha \cdot V_{dd}}{I_{1} \cdot \beta \cdot R_{fb}}.}}}} & {{Eq}\mspace{14mu}(5)}\end{matrix}$

The reference voltages V_(H) and V_(L), may be expressed as:V _(H) =I _(c) ·R ₀ and V _(L) =I _(d) ·R ₀.  Eq (6)

The comparator window size ΔV may be expressed as:

$\begin{matrix}{{{\Delta\; V} = {{V_{H} - V_{L}} = \frac{2{I_{2} \cdot \alpha \cdot V_{dd} \cdot R_{0}}}{I_{1} \cdot \beta \cdot R_{fb}}}},} & {{Eq}\mspace{14mu}(7)}\end{matrix}$

In equation (7), comparator window size ΔV is a function of the powersupply voltage V_(dd) and the feedback resistance R_(fb). Equation (7)may implement equation (2) if β is a function of the integratorcapacitance C.

FIG. 10 shows a process 1000 for operating a dual-mode output circuit.Pulse width modulation (PWM) is performed on an input signal (e.g., witha square wave signal and a single reference voltage) if a PWM mode isselected (block 1012). Pulse density modulation (PDM) is performed onthe input signal (e.g., with adjustable reference voltages) if a PDMmode is selected (block 1014). A class D amplifier is driven with adriver signal obtained by performing PWM or PDM on the input signal(block 1016).

FIGS. 3, 4, 6, 7 and 8 show use of a first-order summing integrator forthe input stage of the PWM and/or PDM modulator. Other circuits may alsobe used for the input stage. For example, the input stage may beimplemented with a higher order integrator, a difference amplifierfollowed by a filter (e.g., as shown in FIG. 5), or some other circuit.

An output circuit may have various configurable circuit components in apulse modulator and/or a class D amplifier. For the pulse modulator,integrator capacitors 126, 526 and 726 may have configurable values thatmay be selected via a control signal. For example, one capacitor valuemay be selected for the PWM mode and another capacitor value may beselected for the PDM mode. Input resistors 124, 524 and 724, feedbackresistors 128, 528 and 728, and/or coupling resistors 142 and 742 mayalso have different configurable values that may be selected via controlsignals. The capacitor and resistor values may be selected to obtain thedesired circuit characteristics.

Different switching frequencies may be supported in the PWM mode byapplying different clock frequencies to a fixed divider or by usingdifferent divider ratios with a fixed clock frequency. For example,switching frequencies of 2.4 MHz down to 300 KHz may be obtained with aclock frequency of 19.2 MHz and divider ratios of 8 to 64, respectively.The nominal comparator window size ΔV may be configurable based on,e.g., the desired switching frequency in the PDM mode. For both PWM andPDM modes, the midscale reference voltage V_(mid) may be configurablebased on, e.g., the power supply voltage V_(dd).

For the class D amplifier, P-FETs 154, 554 and 754 and N-FETs 156, 556and 756 may be designed with configurable slew rates. Drivers 152, 552and 752 may generate the control signals for the P-FETs and N-FETs withdifferent non-overlap durations. These configurable features may beimplemented with appropriate circuits. For example, different slew ratesmay be achieved with multiple FETs in parallel and turning on differentnumbers of FETs. Different non-overlap durations may be achieved bygenerating the control signals with different amounts of delay.

The output drivers described herein may be used for wirelesscommunication devices, handheld devices, gaming devices, computingdevices, computers, laptop computers, consumer electronics devices, etc.An exemplary use of the output drivers for a wireless communicationdevice is described below.

FIG. 11 shows a block diagram of a design of a wireless communicationdevice 1100 in a wireless communication system. Wireless device 1100 maybe a cellular phone, a terminal, a handset, a personal digital assistant(PDA), etc. The wireless communication system may be a Code DivisionMultiple Access (CDMA) system, a Global System for Mobile Communications(GSM) system, etc.

Wireless device 1100 is capable of providing bi-directionalcommunication via a receive path and a transmit path. On the receivepath, signals transmitted by base stations are received by an antenna1112 and provided to a receiver (RCVR) 1114. Receiver 1114 conditionsand digitizes the received signal and provides samples to an applicationspecific integrated circuit (ASIC) 1120 for further processing. On thetransmit path, a transmitter (TMTR) 1116 receives data from ASIC 1120,processes and conditions the data, and generates a modulated signal,which is transmitted via antenna 1112 to the base stations.

ASIC 1120 may include various processing, interface and memory unitssuch as, e.g., a data processor 1122, a reduced instruction setcomputing (RISC) processor 1124, a controller/processor 1126, aninternal memory 1128, an external bus interface (EBI) 1130, and a serialbus interface (SBI) 1132. Data processor 1122 performs processing fordata transmission and reception, e.g., encoding, modulation,demodulation, decoding, etc. RISC processor 1124 may perform varioustypes of processing for wireless device 1100, e.g., processing forvideo, graphics, higher layer applications, etc. Controller/processor1126 may direct the operation of various processing and interface unitswithin ASIC 1120. Internal memory 1128 stores data and/or instructionsfor various units within ASIC 1120. EBI 1130 facilitates transfer ofdata between ASIC 1120 and a main memory 1140. SBI 1132 facilitatescommunication between ASIC 1120 and an ASIC 1150.

ASIC 1150 may include various input/output (I/O) drivers such as, e.g.,an audio driver 1152, a video driver 1154, and an I/O driver 1156. Audiodriver 1152 drives an audio device 1162, which may be a speaker, aheadset, an earpiece, etc. Video driver 1154 drives a display unit 1164,which may be a liquid crystal display (LCD), etc. I/O driver 1156 drivesan I/O device 1166 via an output interface such as Universal Serial Bus(USB), RJ45 or Ethernet, etc. Audio driver 1152, video driver 1154,and/or I/O driver 1156 may be implemented with the output circuitsdescribed herein.

ASIC 1120 may be a Mobile Station Modem (MSM) from QualcommIncorporated. ASIC 1150 may be a Power Management IC (PMIC) fromQualcomm Incorporated. ASICs 1120 and 1150 may also be other ASICs fromother manufacturers.

The output circuits described herein may be fabricated in various ICprocesses such as N-channel metal oxide semiconductor (N-MOS), P-channelMOS (P-MOS), complementary MOS (CMOS), bipolar, bipolar-CMOS (Bi-CMOS),gallium arsenide (GaAs), etc. The output circuits may also be fabricatedon various types of IC such as analog ICs, digital ICs, radio frequencyICs (RFICs), etc.

An output circuit described herein may be implemented on a device. Thedevice may be (i) a stand-alone IC such as a driver IC or a PMIC, (ii)an ASIC such as a Mobile Station Modem (MSM), (iii) a circuit that maybe embedded within other devices, (iv) a cellular phone, wirelessdevice, handset, or mobile unit, (vi) etc.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples described herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. A device comprising: a comparator configured toreceive an analog signal and adjustable reference voltages, to comparethe analog signal against the adjustable reference voltages, and togenerate a driver signal with pulse density modulation (PDM); a class Damplifier configured to receive the driver signal and generate an outputsignal; and a reference generator configured to generate the adjustablereference voltages.
 2. The device of claim 1, further comprising: anintegrator configured to receive, sum and integrate an input signal andthe output signal and to provide the analog signal.
 3. The device ofclaim 2, wherein the integrator comprises an amplifier having aninverting input, a non-inverting input, and an output, a first resistorcoupling the input signal to the inverting input of the amplifier, and asecond resistor coupling the output signal to the inverting input of theamplifier.
 4. The device of claim 3, wherein the reference generator isconfigured to generate first and second reference voltages having adifference that is inversely related to a value of the second resistor.5. The device of claim 1, wherein the reference generator is configuredto generate first and second reference voltages having a difference thatis related to a power supply voltage.
 6. The device of claim 1, whereinthe reference generator is configured to generate first and secondreference voltages having a difference that is determined by the inputsignal.
 7. The device of claim 1, wherein the reference generator isconfigured to generate a first reference voltage when the driver signalis at logic high and to generate a second reference voltage when thedriver signal is at logic low, the second reference voltage being higherthan the first reference voltage.
 8. The device of claim 1, wherein thereference generator is configured to generate the adjustable referencevoltages to reduce variation in switching frequency of the driver signaldue to variations in integrated circuit (IC) process, power supplyvoltage, and temperature.
 9. A device comprising: a comparator meansconfigured to receive an analog signal and adjustable referencevoltages, to compare the analog signal against the adjustable referencevoltages, and to generate a driver signal with pulse density modulation(PDM); a class D amplifier means configured to receive the driver signaland generate an output signal; and a reference generator meansconfigured to generate the adjustable reference voltages.
 10. A methodcomprising: generating adjustable reference voltages; receiving ananalog signal and the adjustable reference voltages; comparing theanalog signal against the adjustable reference voltages; generating adriver signal with pulse density modulation (PDM) based on thecomparison; receiving the driver signal; and generating an outputsignal.